Broadening of solar wind strahl pitch‐angles by the electron/electron instability: Particle‐in‐cell simulations

Solar wind electrons in the suprathermal energy range from about 70 eV to greater than 1 keV usually are observed as two distinct, relatively tenuous components: an approximately isotropic halo and a strongly anisotropic “strahl” directed along the magnetic field. This manuscript describes a two‐dimensional electromagnetic particle‐in‐cell simulation in a magnetized, homogeneous, collisionless plasma with such a two‐component representation of suprathermal electrons. In this computation the electrostatic electron/electron instability is driven by a strahl with a large average speed along the background magnetic field B o . The simulation demonstrates that this instability leads to substantial strahl velocity scattering perpendicular to B o . The most important result here is that, for strahl speeds large compared to the halo thermal speed, after instability saturation the width of the electron pitch‐angle distribution exhibits a maximum as a function of electron energy.